1. Technical Field
The present invention relates to a feedforward control apparatus capable of smoothly stably operating a servomotor.
2. Background Art
Generally known is a servo system for controlling a servomotor mounted as a drive source on various machines, such as machine tools, robots, etc., so that the deviation between a command motor position and an actual motor position is brought into zero. If there occurs a follow-up delay with respect to a position command in this servo system, the positional deviation increases. In machine tools, for example, this entails errors in work shape. Particularly, at the time of high-speed machining in which the position command quickly changes, such errors are liable to be caused.
In this respect, it is conventionally known to perform phase-lead compensation for the position command, i.e., position feedforward control, by adding a differential value of the position command to a speed command corresponding to the positional deviation, so as to eliminate a response delay of the servo system with respect to the position command, thereby preventing control errors, e.g., the work shape errors entailed in the high-speed machining.
The speed command, however, contains more high-frequency components when it is phase-lead-compensated than when it is not. On the other hand, a speed control loop of the conventional servo system for delivering a current command corresponding to the deviation between the speed command and an actual speed is not responsive enough to the high-frequency components of the speed command. According to the conventional servo system, therefore, the positional deviation changes in undulations. In other words, the action of the servomotor, and hence, of a mechanical system which uses the motor as its drive source, becomes unstable, and their vibration cannot be easily damped.
Also known is a servo system of another type in which a position command delivered from a numerical control apparatus is subjected to acceleration/deceleration processing so that the position command can change smoothly. Despite such acceleration/deceleration processing, however, the positional deviation is still subject to undulations, so that the motor may possibly be shocked.